$300 sounds like a great price. Unless the Rigid Summicron is a "user" or needs work, expect to kick in some cash with your first Summarit. It would probably trade even for a similar condition collapsible Summicron. I prefer the collapsible over my Type 1 Rigid and Type 2 Rigid. The optics module of the Type 1 and DR are identical.
Once you get beyond 5 feet from the subject, or stop down to F4- the Summarit Swirlies calm down.
I LOVE using this lens wide-open and close-up, I LOVE making the world revolve around my daughter.